1. Field of the Invention
The present invention relates to novel solutions of linear aromatic poly(imideamides) which are both clear at room temperature (23.degree. C.) and stable in storage at the same temperature, and to the production of films and varnishes therefrom which have a color, as determined by the luminance value (as defined below) of a sample thereof which has a thickness of 10 to 100 .mu.m and is deposited onto a stainless steel substrate, ranging from 10 to 50.
The present invention also relates to a process for preparing poly(imideamide) solutions exhibiting the properties indicated above.
In the international XYZ trichromatic system defined by the International Commission on Illumination in 1931, the Y component, or luminance value, indicates the physiological impression of luminosity (ranging from dark to light) of a sample. A luminance value of zero corresponds to the color black and a value of 100 corresponds to the color perfect white. In practice, the luminance value is determined by measuring the scattered reflection of the sample which has a thickness of 10 to 100 .mu.m and is deposited on a stainless steel substrate, in the green region of the spectrum (wavelength centered at 550 nm) with the aid of a filter colorimeter of the Elrepho Zeiss trademark.
2. Description of the Prior Art
FR-A-2,079,785 describes solutions based on poly(imideamide) in N-methylpyrrolidone, but such solutions have an intense brown color which remains in the articles produced from these solutions and which consequently limits the use of such articles.
FR-A-2,627,497 relates to reducing such objectionable discoloration by (a) conducting the polycondensation reaction producing the poly(imideamide) in a solvent consisting of anhydrous .gamma.-butyrolactone (.gamma.-BL), free from butyl alcohol and employed in an amount such that the polymer concentration upon completion of the polycondensation reaction (expressed as the percentage by weight of polymer in the total of polymer+solvent) ranges from 12% to 39%, and then (b), at the end of said polycondensation reaction, carrying out a hot dilution of the reaction solution at a temperature ranging from 120.degree. C. to 165.degree. C. with an anhydrous aprotic polar solvent such as N-methylpyrrolidone, dimethylacetamide, dimethylformamide or tetramethyl urea, said diluting solvent being employed in an amount such that, on the one hand, the final concentration of the poly(imideamide) in the total amount of polymer+polycondensation solvent+diluting solvent ranges from 5% to 30% by weight and, on the other, the weight ratio of polycondensation solvent/diluting solvent ranges from 40/60 to 80/20. From said FR-A-2,627,497, and in particular Example 5 thereof, it appears essential to conduct the polycondensation reaction in .gamma.-butyrolactone for the desired decrease in discoloration to be attained.
Thus, the process described in the abovementioned FR-A-2,627,497 presents the disadvantage of requiring a second hot dilution stage using, moreover, a second solvent which cannot advantageously be the same as that used in the polycondensation, to produce poly(imideamide) solutions which do not precipitate and which remain clear or stable when they are stored at room temperature (23.degree. C.) (cf. Example 18). Such additional second stage appreciably increases the cost of manufacture of the desired solution.